The negatively charged nitrogen-vacancy center in diamond (NV) is an optically addressable room-temperature solid-state spin system with phase coherence times approaching one second. The NV center is electric-field sensitive through the Stark shift, which changes its electron spin energy levels in applied field. It has applications in many fields, including biology, where it is used as a fluorescent probe; quantum information, where it is used as a quantum bit; and sensing, where offers the ability to sense temperature, time, and electromagnetic fields with high precision.
For example, NV centers have been used to sense electric field with a spin echo technique. However, spin echo techniques involve both precise alignment of an external magnetic field with the NV orientation to achieve electric field sensitivity and a repetitive, phased-locked alternating-current (AC) electric field to achieve the highest resolution. (In general, alignment may be necessary for electric-sensitive spin echos, but not for spin echos.) As a result, these schemes are impractical for use with nanodiamonds which have random orientations in a tissue. Furthermore, to sense aperiodic electric fields, this scheme is limited to a non-dynamically decoupled phase acquisition time T2* T2* which is many orders of magnitude lower than what can be achieved using decoupling sequences. Since the magnetic field must be precisely aligned, fluctuations in field direction or off-axis magnetic noise can greatly diminish the sensitivity.
NV centers have also been used to sense DC magnetic fields using Ramsey interferometry, single-frequency AC magnetic fields using Hahn echo techniques, and general AC magnetic fields using repetitive dynamic decoupling sequences, such as the Carr Purcell Meiboom Gill (CPMG)-N dynamical decoupling sequence or the XY8·N dynamical decoupling sequence. These sequences rely on a differential phase acquired by different Sz components of the spin ½ NV system, using a transition between the ms=0 and a single ms=±1 state of the NV ground state spin triplet.
The transition between the ms=0 and a single ms=±1 state of the NV ground state spin triplet is pressure dependent through its relation to the strain of the diamond crystal. As such, measurement of the resonance frequency of this transition can give a readout of local pressure, with accuracy determined by the spin properties of the diamond as well as the specific method of spin probing. Two schemes use a π/4−π−/4 pulse sequence that addresses a double quantum transition between the ms=0 and both the degenerate ms=±1 and ms=−1 levels. This sequence produces a signal that depends only on Sz2, and therefore can sense local temperature or the frequency detuning of the driving microwave while providing immunity to other environmental effects, significantly including magnetic fields.